This invention relates to transport of time-division multiplexed data traffic in a synchronous communication system.
Fixed-rate data traffic can be transported using time-division multiplexing (TDM) of synchronous data frames. The approach to multiplexing data traffic in conventional SONET/SDH (Synchronous Optical Network/Synchronous Digital Hierarchy) systems is an example of such a TDM approach. SONET/SDH standards were developed as an evolution of legacy copper based transmission equipment to serve as a next generation/broadband transport of voice traffic over fiber optic infrastructure.
The first generation of digital transmission equipment used physical layer technologies that were encompassed under three regional digital signal hierarchies. The North American hierarchy consists of DS0 (64 kb/s), DS1 (1.544 Mb/s), DS1c (3.152 Mb/s), DS2 (6.312 Mb/s), DS3 (44.736 Mb/s), DS3C (91.035 Mb/s) and DS4 (274.176 Mb/s) signals. The European hierarchy consists of E0 (64 kb/s), E1 (2.048 Mb/s), E2, E3 and E4 signals. The majority of the broadband optical fiber communications are based today on the SONET/SDH family of standards (SDH is essentially the international standard corresponding to SONET). The standards provide mechanisms to transport circuit switched traffic streams within higher speed SONET “pipes,” which are aggregated streams of multiplexed low speed traffic. A series of Bellcore and ANSI specifications define data formats of payload containers (typically referred to as virtual tributaries, or VTs) to carry legacy traffic rates (DS1, DS1C, DS2 and DS3, of what is known as the PDH, or the Plesiochronous Digital Hierarchy) in higher speed synchronous communication on the optical links.
Communication according to the SONET standard makes use of a ring architecture in which a number of communication nodes are connected by optical links to form a ring. A SONET ring typically has a number of nodes each of which includes an add/drop multiplexer (ADM). Each of the nodes is coupled to two neighboring nodes by optical paths. Communication passes around the ring in a series of synchronous fixed-length data frames formatted according to a Synchronous Transport Signal (STS) standard. Each ADM is configured to pass a portion of the communication on the ring without modifying it, to extract (“drop”) a portion of the communication destined for that node, and to “add” outbound communication leaving the node to the optical path. The granularity of adds and drops in ADMs is typically an STS-1, which carries a DS3 rate data stream. The dropped and added communication passes between the ADM and local communication equipment, such as a multiplexer, which multiplexes a number of separate traffic streams. For example, an added or dropped communication stream may be a 1.5 Mb/s (DS1) data stream on which separate 64 kb/s (DS0) telephone channels that are multiplexed. The DS1 data stream is multiplexed onto (added to) the optical path and passed between particular nodes on the SONET ring. Typically, a SONET ring is provisioned to provide fixed-rate bidirectional communication streams, also known as virtual paths, between different ADMs on the ring. The virtual paths couple the separate communication streams that enter and leave the SONET ring at the ADMs. In operation, the virtual paths coupling different communication streams, including their allocated data rates, typically remain fixed for long periods of time.
The process of multiplexing standard rate data streams into higher rate streams is a basic feature of SONET communication. Multiplexed data streams pass between nodes in a SONET ring at particular data rates. These rates form a hierarchy of standard rate streams that are defined as part of the SONET standards. At the lowest rates, a VT1.5 virtual tributary supports a 1.5 Mb/s data rate. This is the data rate of a common DS1 (T1) service, and can support up to 24 separate 64 kb/s (DS0) data streams. A VT2 virtual tributary supports approximately 2 Mb/s data, and a VT6 virtual tributary approximately supports 6 Mb/s. These virtual tributaries are typically the smallest units of communication that are added or dropped at an ADM. Virtual tributaries can be combined into a virtual tributary (VT) group, which can consist of 4 VT1.5, 2 VT2 or 1 VT6 virtual tributaries, and entire VT groups can be added and dropped at an ADM.
In different configurations of SONET rings, communication on the optical links can be at different data rates and use various forms of multiplexing. In one mode, a series of synchronous STS-1 frames includes a series of Synchronous Payload Envelopes (SPEs), which can be used to carry 45 Mb/s data between the SONET nodes. The series of SPEs can carry a raw data rate of 45 Mb/s or can be used to carry seven VT groups, each of which can multiplex multiple equal-size virtual tributaries. The STS-1 frame adds control and overhead data to the SPE for transmission. The STS-1 frame can be optically encoded as an OC-1 signal for transmission over an optical link, or multiplexed as three STS-1 frames to an STS-3 frame and optically encoded as an OC-3 signal for transmission over a higher capacity optical link. An STS-3 frame can alternatively carry a concatenated STS3c payload envelope, which carries data at 150 Mb/s. The STS-3 frame can multiplexed still further, for instance four STS-3 frames to a STS-12 frame, which is in turn optically encoded as an OC-12 signal. Likewise, a SONET frame could be a concatenated STS-48c frame encoded as an OC48 optical signal, and a single payload envelope accounts for the entire OC48 payload.
SONET uses pointers in the frames to compensate for frequency and phase variations of the clocks used to transmit and receive data. Each STS-1 frame includes a pointer (H1,H2 bytes) in the transport overhead (TOH) of that frame to the offset of start of the SPE in that frame. When multiple sequences of STS-1 frames are dropped at an ADM, the ADM determines start of each of the SPEs separately based on the offsets in the respective STS-1 frame. When VTs are carried within an SPE, each VT can also include a VT payload pointer (V1,V2 bytes), which specifies the alignment of the VT within the SPE. In general, the phase of the incoming SPEs have no particular relationship to the phase of the synchronous STS frames.
Clocking in SONET networks is typically organized with a master-slave relationship with clocks of the higher-level nodes feeding the timing signals to lower-level nodes. The internal clock of a SONET node can derive its timing from an external source, such as a Building Integrated Timing Supply (BITS), in which case it serves as a master for other SONET nodes to which it is connected. At slave nodes, the internal clock is derived using “line timing” from an incoming OC-n signal. Typically, a SONET ring is configured to have one node timed to an external source, and the remaining nodes timed off the ring as slaves.
Although all nodes in a SONET ring are timed to a common source, there may nevertheless be small frequency differences (jitter/wander), which result due to several reasons, including span lengths between nodes. To accommodate these small frequency differences between an incoming signal and an outgoing signal, the SONET pointer mechanisms provides positive and negative justification opportunities. The frequency justification is particularly applicable when multiplexing lower rate signals into a higher rate synchronous signal.
Pointer processing is also used to account for differences in phase between the receive and transmit frames. When a payload is passed from the input to the output of a node, a phase adjustment between the payload is performed by adjusting the value contained in the H1-H2 bytes in the TOH of outbound STS frame. Hence, if the phase of the incoming STS frame is different from the transmitted frame, the SPE within the passed-through STS frame is multiplexed from the receive frame into the appropriate location within the transmit frame, and the H1-H2 bytes within the transmitted STS frame's TOH are adjusted to reflect the new position of the SPE. Therefore, the incoming SPE is transmitted to the outgoing SONET frame with minimum delay, even if the phase difference of the incoming and outgoing STS frames are substantially different. When the payloads of multiple STS-1 frames are multiplexed into a larger frame, traditional SONET ADMs process the pointers for each STS-1 payload within the multiplexed frame independently. For instance, in an OC-48 SONET frame in which 48 STS-1 frames are multiplexed, the ADM performs separate pointer processing on each of the 48 STS-1 frames. Note that the ADM performs pointer processing for all the STS-1 frames, not only those involved in add or drop functions at that node. Typically all the outbound STS-1 frames are synchronized to a common phase, and the H1-H2 pointers are manipulated in all outbound STS-1 frames to indicate the offsets of the SPEs in those frames.
A traditional SONET ADM breaks up a synchronous STS-n frame into channels of fixed/integral granularity, typically STS-1 or STS-3. The multiplexing/demultiplexing mechanisms are broken up into two stages. First, individual STS-n channels are added/dropped/passed-through at each node. Each of the dropped STS-n channels are broken down further to identify the particular VTs which need to be extracted. This requires either an entire STS-n channel to be added/dropped at a particular node off a ring, or additional VT cross-connect logic is necessary at the back-end of the STS cross-connect/add/drop logic to multiplex lower speed streams into an STS-n. This can cause severe fragmentation and under-utilization of a SONET frame, particularly as SONET scales to higher bandwidths.